This invention relates to a bias circuit for power amplifiers, especially a single-ended push-pull (noted as SEPP hereafter in this specification) amplifier.
Conventionally, in a class A SEPP amplifier, a large DC bias current is passed to the final stage of an SEPP amplifier at all times in order to obtain class A operation. Consequently, there is a shortcoming that effeciency at low output power is extremely poor and a large capacity power supply and a large thermal radiator are necessary.
There is also known a class AB SEPP amplifier in which the DC bias current is passed to a certain degree, thus operating at class A up to a fixed output power and not allowing class A operation when the output is higher than the fixed output power. However, this class AB SEPP amplifier has a shortcoming that no class A operation is available at an output higher than the fixed output power.
Also, there is known a class B SEPP amplifier designed to pass DC bias current forcibly to the output transistors even at the normal cut-off time of the class B SEPP amplifier so that the output transistors comprising the output stage are not cut off. Although this class B SEPP amplifier may be called a class A amplifier in the sense that the output transistors of the output stage do not become cut-of, but it has a shortcoming that the current waveform of each transistor is different from that of the class A amplifier.